1. Field of the Invention
The present invention relates to a method for preparing an .alpha., .beta.-unsaturated nitrite in which .alpha.-oxycarboxylic acid amide is used as a starting material.
2. Description of the Prior Art
There have been known several methods for preparing .alpha., .beta.-unsaturated nitriles. One of such methods comprises subjecting an .alpha.-oxynitrile to a dehydration reaction, the .alpha.-oxynitrile being able to be easily and industrially synthesized from an aldehyde or a ketone and HCN in high yield. For instance, there has been known a method for preparing acrylonitrile which comprises dehydrating lactonitrile synthesized from acetaldehyde and HCN in the presence of phosphoric acid.
However, in this method, the reaction temperature is very high in the order of 600.degree. to 700.degree. C. and the lactonitrile as the starting material has very low thermal stability. Therefore, it is needed to rapidly raise the reaction temperature to a desired level. For this reason, it is necessary to adopt a special method in which the starting material containing phosphoric acid is sprayed into a reactor filled with a superheated gas having a temperature of 1,000.degree. C. or higher. The selectivity of acrylonitrile in such a method is about 70% even if the method is designed as described above.
Recently, it has been found that .alpha.-oxynitrile can easily be converted into .alpha.-oxycarboxylic acid amide in high yield by subjecting it to hydration in the presence of a manganese dioxide catalyst (see U.S. Pat. No. 3,366,639).
There has not yet been proposed any method for directly preparing .alpha., .beta.-unsaturated nitriles from .alpha.-oxycarboxylic acid amides thus obtained at one stage, but as an indirect multistage method; U.S. Pat. No. 2,183,357 discloses a method which comprises acylating the hydroxyl and amino groups of an .alpha.-oxycarboxylic acid amide and then subjecting the acylated product to thermal decomposition. The .alpha., .beta.-unsaturated nitrite is one of the plurality of products formed in such a method, but the formation thereof requires the use of a high temperature; of the order of 500.degree. C. or higher. In addition, U.S. Pat. No. 2,859,240 discloses a method for preparing an .alpha., .beta.-unsaturated nitrite in which an ammonium .alpha.-oxycarboxylate obtained by further hydrating the corresponding .alpha.-oxycarboxylic acid amide is used as a starting material. It is confirmed that an unsaturated nitrite is present in a distillate of the product of this method, but the formation thereof likewise requires the use of a high reaction temperature in the order of 454.degree. to 510.degree. C.
Moreover, U.S. Pat. No. 4,161,609 discloses a method for synthesizing an .alpha., .beta.-unsaturated nitrite in which an .alpha., .beta.-unsaturated amide is used as a starting material. However, since the method is performed with the coexistence of alcohols, the main products of the method are .alpha., .beta.-unsaturated esters and the yield of the .alpha., .beta.-unsaturated nitrile in the method is correspondingly very low. Thus, this method is not suitable for preparing an .alpha., .beta.-unsaturated nitrile.
As has been discussed above in detail, there has not yet been known any synthetic method for directly obtaining an .alpha., .beta.-unsaturated nitrile using an .alpha.-oxycarboxylic acid amide per se as a starting material.
Moreover, U.S. Pat. No. 4,464,539 discloses a method for preparing .alpha., .beta.-unsaturated esters and/or .alpha., .beta.-unsaturated carboxylic acids by reacting an .alpha.-oxycarboxylic amide with water and/or an aliphatic alcohol in the presence of a solid acid catalyst at temperatures of 150.degree. C. or more. The reaction conditions set forth in the Examples using water only are such that the mole ratio of water to .alpha.-oxycarboxylic acid amide is the range of 17-20 in case of using water only and the reaction product is only an .alpha., .beta.-unsaturated carboxylic acid; the formation of .alpha., .beta.-unsaturated nitrile is not observed.